As electronic equipment has become higher in performance and smaller in size and weight, electronic devices have become higher in performance and smaller in size and weight, requiring materials forming electronic devices to have higher performance and as small size as a nano level. For instance, magnetic particles coated on magnetic tapes are required to have as small size as a nano level and improved magnetization to achieve a higher magnetic recording density.
Magnetic nano-particles are mainly produced by liquid-phase synthesis methods such as a coprecipitation method, a hydrothermal synthesis method, etc. Magnetic nano-particles obtained by the liquid-phase methods are oxide particles such as ferrite, magnetite, etc. Recently, methods utilizing the thermal decomposition of organometallic compounds are used to produce, for instance, Fe nano-particles from Fe(CO)5.
Because metals are more magnetized than oxide-type magnetic particles, metal particles are expected to be used in industrial applications. For instance, metal Fe has saturation magnetization of 218 A·m2/kg, much larger than that of iron oxides. Accordingly, the metal Fe has excellent magnetic response, enabling larger signal intensity. However, metal particles such as the metal Fe, etc. are easily oxidized. For instance, when metals are formed into fine particles of 100 μm or less, particularly 1 μm or less, they tend to be vigorously burned in the air because of increased specific surface areas, resulting in difficulty in handling in a dry state. Accordingly, oxide particles such as ferrite, magnetite, etc. have widely been used.
In handling dry metal particles, it is indispensable that the metal particles are coated lest that they are in direct contact with air (oxygen). However, surface-coating with metal oxides results in appreciable oxidation of metals (JP 2000-30920 A).
JP 9-143502A proposes a method for producing graphite-coated metal particles comprising the steps of mixing carbonaceous particles such as carbon black, natural graphite, etc. with simple metal particles or metal compound particles such as metal oxides, metal carbides, metal salts, etc., heating them at 1600-2800° C. in an inert gas atmosphere, and cooling them at a speed of 45° C./minute or less. However, because metal-containing particles are heat-treated at extremely high temperature of 1600-2800° C. in this method, metal particles are likely sintered. In addition, the coating of metal particles with graphite disadvantageously suffers low productivity.
Known as a coating method free from this problem is the coating of metal particles with boron nitride (BN) [see, for instance, International Journal of Inorganic Materials 3, p. 597 (2001)]. BN is a material used for crucibles, etc., having as high a melting point as 3000° C., excellent thermal stability, low reactivity with metals, and insulation. Methods of coating metal particles with BN include (1) a method of heating a mixture of metal particles and B particles in a nitrogen atmosphere by arc discharge; (2) a method of heating a mixture of metal particles and B particles in a mixed atmosphere of hydrogen and ammonia; and (3) a method of heat-treating a mixture of metal nitrate, urea and boric acid in a hydrogen atmosphere. Particularly the methods (2) and (3) are expected to avoid the sintering of metal particles, because heating is conducted at as low temperature as 1000° C. However, BN-coated metal particles are extremely expensive.
In addition, because graphite has a structure in which graphene sheets are laminated, graphite covering spherical metal particles inevitably has lattice defects. Boron nitride similarly has a laminated structure, failing to provide a completely crystalline coating layer. Coatings having these defects are unsatisfactory in applications needing high corrosion resistance, such as magnetic beads, etc. Accordingly, fine metal particles having high corrosion resistance, and a method for producing such fine metal particles with excellent industrial productivity at low cost are desired.